Currently 22-23 magnitude could be the limit to make a spectrum for ground based 10m class telescopes, 21-22 magnitude may be easy for them.
4m class telescopes could possibly handle 20 magnitude, I am not sure.
2m could be appropriate for stars brighter than 16magnitude generally.
What about the infrared bands: J H K W1 W2 respectively? Are there any empirical values or references?